What it is and where it lives
The Siemens SIRIUS 3RU2126-4DC1 is a thermal overload relay — the thing that sits between your contactor and the motor, watching for a locked rotor or a sustained overcurrent before the windings cook. It's a Class 10 trip, meaning it will open the circuit within 10 seconds at 7.2× the set current. That's fast enough to protect a standard squirrel-cage induction motor driving a conveyor, a pump, or a kiln fan, but not so fast that it nuisance-trips on a normal start. Rated insulation voltage is 690 V, and it carries motor ratings across common line voltages: 11 kW at 400 V, 15 kW at 500 V, 22 kW at 690 V. At 480 V and 600 V it's rated 25 A. That covers most 400 V class motors up to about 15 kW and 690 V class up to 22 kW.
Surviving the environment
This relay is built for the heat and the grit. Operating temperature range is -40 to +70 °C, with temperature compensation active from -40 to +60 °C. That means the bimetallic strip inside still tracks the motor's true thermal state even when the panel is sitting next to a preheater tower or a clinker cooler. Storage and transport range is -55 to +80 °C. Mounts in any position, stand-alone installation. The S0 size — 45 mm wide, 114 mm tall, 95 mm deep — fits a standard DIN rail panel. Spring-loaded terminals accept 2× 0.5 to 2.5 mm² solid or stranded, so you're not fighting screw terminals when the panel is caked with dust. The auxiliary switch is integrated, no add-on block needed.
Auxiliary contact ratings — what they actually switch
The integrated auxiliary contact handles common control voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V. At 125 V DC it's 0.22 A, at 220 V DC it's 0.11 A. That's enough to drive a PLC input or a contactor coil in the same panel. Per-pole power dissipation is 2.7 W — negligible for thermal budgeting in a ventilated enclosure.
Lifecycle and sourcing reality
MTTF under high demand rate is 2,280 years.